Polyisocyanurate foam (i.e., PIR board stock) has been widely used to insulate roofs and walls of commercial and industrial buildings for many decades due to its excellent thermal insulation, flame resistance, and mechanical properties. The main reason for its excellent flame resistance is the formation of isocyanurate trimer during the foaming process. However, in order to pass the fire tests required by building code, such as the ASTM E-84 test, extra fire retardants are still needed. Halogenated fire retardants, such as TCPP (tris(2-chloroisopropyl)phosphate), are commonly used in PIR board stock. Halogenated fire retardants, especially TCPP, are very effective fire retardants due to their dual functions of gas phase radical scavenging and char formation in the solid phase. However, some insulation specifiers have expressed an interest in using materials which do not contain TCPP or other halogenated fire-retardants.